TITLE: GCN CIRCULAR NUMBER: 42856 SUBJECT: GRB 251126A: ULL-ASTRO-MASTER detection of the optical afterglow with LCO 1-m telescope at McDonald Observatory DATE: 25/11/27 06:40:21 GMT FROM: Ismael Perez-Fournon at Instituto de Astrofisica de Canarias <ipf@iac.es> M. Pulido-Torres, J. Basurto Merino, P.G. Berdayes, A. Caballero-Almagro, A. Cerón, M. Contreras, F. Díaz-Segado, T. Ferrer-Laviña, B. Gandolfi, V. Ghiraldo, J. Hernández Fung, L. Juliá-Maroto, E. Lekaroz-Urriza, M. Manzano García, E. Mejía-Martínez, J. Prieto Polo, M. Quintana-Ansaldo, A. Schenone-Zanuzzi, A. Selezneva, T. Tundidor Rodríguez, E. Urquijo-Rodríguez (all ULL), M. Abdul-Masih (IAC and ULL), and I. Pérez-Fournon (IAC and ULL). Following the detection of the Swift GRB 251126A (Caputo et al., GCN Circ. 42843; Evans et al., GCN Circ. 42845; and Goad et al., GCN Circ. 42851), we observed the field with one of the two Las Cumbres Observatory (LCO) 1-m telescopes equipped with Sinistro cameras located at the LCO node at McDonald Observatory, Texas. The observation, a single exposure of 300 sec in the SDSS r' filter, started on 2025-11-27 at 04:41:19 UT, about 9.51 hours after the Swift trigger. The optical counterpart first detected by the GROWTH-India Telescope (GIT) (Swain et al., GCN Circ. 42844) is clearly detected in our image with a magnitude of r' = 20.86 +/- 0.13 (AB), calibrated against PanSTARRS-1 DR2 stars and not corrected for Galactic extinction. Our result is consistent with other optical observations: Swain et al. (GCN Circ. 42844), Lipunov et al. (GCN Circ. 42847), Fu et al. (GCN Circ. 42848), Reguitti et al. (GCN Circ. 42849), Broens et al. (GCN Circ. 42850), and Angulo et al. (GCN Circ. 42855). This work makes use of observations from the Las Cumbres Observatory global telescope network (LCO program IAC2025B-010). These observations are part of a course in Astrophysical Techniques of the Master in Astrophysics of the Astrophysics Department of the University of La Laguna in collaboration with the Instituto de Astrofísica de Canarias (La Laguna, Tenerife, Canary Islands, Spain). This work made use of the Astro-COLIBRI platform (P. Reichherzer et al. 2021, ApJS, 256, 5). View this GCN Circular online at https://gcn.nasa.gov/circulars/42856. --- To unsubscribe, open this link in a web browser: https://gcn.nasa.gov/unsubscribe/eyJhbGciOiJIUzI1NiJ9.eyJlbWFpbCI6InZzbmV0LW...